Page 27 - MetalForming August 2010

steel could be 0.08 to greater than 0.25. Two factors generate this wide range. First, pots of molten zinc generally produce the same topography as the zinc solidifies on the steel. Electro-galvanized steel is coated by lines of elec- trolytic plating cells. There are many different designs for these cells. Second, the electro-galvanized steel coils are not given a temper pass as the final process. That means the topography of the zinc created during plating is the topography shipped to the customer. Scanning electron photomicrographs revealed that different coating lines at different steel companies produce a dif- ferent “signature topography” that not only correlates to the steel company but also to a narrow range of COF values.
The study above was initiated when eight coils of electro-galvanized steel were received by a stamping plant. Four coils made good stampings, the other four caused breakage. Tracing the process history of these four coils showed that all eight base coils had been processed at the same time from the same heat of steel. Four coils were then electro-coated by the company that made the steel —which was the normal procedure and the one used for the final tryout steel. These four coils made satisfactory parts. Due to limited capacity at the company, the other four coils were sent to an outside coil-plat- ing company. All four of those coils generated failed stampings. Again, scanning electron photomicrographs revealed a completely different zinc sur- face topography between the successful coils and the failure coils.
An electro-galvanized steel sample that had an unusual failure event was received by a product application center. One side of the stamping (coil left edge) had failures, while the other side of the stamping (coil right edge) had severe buckles. The COF values were determined across the width of the coil using the draw bead simulator. The COF for the left edge was 0.25, which restricted material flow into the die and caused the failure. The COF for the right edge was 0.15, which allowed excess material to slide into the die to cre- ate buckles. Again, the scanning electron photomicrographs showed a major difference across the width of the coil. Without data highlighting the differ- ences in COF, major changes to the tooling (grinding and welding) probably would have taken place to make that coil run successfully. Unfortunately, if the next coil was normal, the whole die modification would have to be reversed. Getting a die back to its “sweet spot” is not easy after undocumented random changes.
In the old days it was nice to know something about the steel you were using. With the strict, new requirements on dimensional consistency, min- imum thickness after forming, cosmetic appearance and other stamping specifications, maximum information about your steel type and processing his- tory is mandatory for running a competitive press shop. MF
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